Sensors for sensing the concentration of a chemical agent are advantageous in a variety of industrial and medical applications. One such application is sensing a concentration of a gaseous or vaporous decontaminating agent (e.g., vaporized hydrogen peroxide) used for decontamination of medical instruments isolators, rooms, etc. Successful decontamination requires exposure to a predetermined concentration of a decontaminating agent over a predetermined period of time. Therefore, accurate measurement of the concentration of the decontaminating agent is advantageous to achieve adequate decontamination and efficient utilization of the decontaminating agent.
In order to insure accuracy, sensors used for measuring, i.e., sensing, the concentration of decontaminating agents are periodically calibrated. According to conventional calibration methods, a sensor for sensing the concentration of decontaminating agents is calibrated by exposing the sensor to a sample of the decontaminating agent at a known concentration. After the sensor is exposed to the sample, the sensor is adjusted to provide a signal that is indicative of the sensed concentration. In order to calibrate the sensor at different concentrations, the calibration method can be repeated using a series of samples having different known concentrations.
The foregoing calibration method is not well suited for calibrating sensors used to determine the concentration of unstable chemical agents such as vaporized hydrogen peroxide. In this regard, vaporized hydrogen peroxide will decompose into oxygen and water. Because vaporized hydrogen peroxide decomposes, a known, stable concentration of vaporized hydrogen peroxide may not be reliably prepared for use in calibrating a sensor using conventional methods. Accordingly, other methods of calibration have been developed to calibrate sensors used to determine the concentration of vaporized hydrogen peroxide, as will be described below.
Typical sensors used for determining the concentration of vaporized hydrogen peroxide include infrared (IR) sensors (e.g., near infrared (NIR) sensors) and electrochemical sensors.
A conventional IR sensor includes a source of infrared radiation (“IR source”) and an infrared detector that are located a fixed distance apart. An optical filter is disposed either in front of the source of infrared radiation or the IR detector to screen out all radiation except for the wavelength that is absorbed by vaporized hydrogen peroxide. Vaporized hydrogen peroxide passes between the IR source and the IR detector.
The amount of IR radiation (provided by the IR source) that is absorbed by the vaporized hydrogen peroxide is proportional to the concentration of vaporized hydrogen peroxide. Accordingly, the IR sensor generates a signal that is indicative of the concentration of vaporized hydrogen peroxide based upon the proportion of IR radiation (provided by the IR source) that is received by the IR detector.
A conventional IR sensor is typically calibrated using an optical filter that is placed between the IR source and the IR detector. The optical filter blocks some of the IR radiation at the same wavelength absorbed by the vaporized hydrogen peroxide. In this regard, the optical filter simulates the presence of vaporized hydrogen peroxide at a known concentration. The IR sensor is adjusted such that it provides a signal indicative of the concentration of vaporized hydrogen peroxide simulated by the optical filter. One drawback to using an optical filter for calibration of IR sensors is that a range of different optical filters is required to calibrate an IR sensor over a range of concentrations.
A conventional electrochemical sensor reacts with vaporized hydrogen peroxide to produce an electrical signal proportional to the concentration of the vaporized hydrogen peroxide. A typical electrochemical sensor includes a first electrode and a second electrode that are connected by a resistor. A thin layer of electrolyte separates the first and second electrodes. The first electrode is formed of a material that is reactive with vaporized hydrogen peroxide.
Vaporized hydrogen peroxide that comes in contact with the reactive material of the first electrode participates in a chemical reaction that generates a current. The current flows between the two electrodes and is proportional to the concentration of vaporized hydrogen peroxide. Accordingly, the amount of current produced by the electrochemical sensor is indicative of the concentration of vaporized hydrogen peroxide.
As indicated above, vaporized hydrogen peroxide is an unstable vapor and will decompose over time. Therefore, a typical method for calibrating an electrochemical sensor for determining the concentration of vaporized hydrogen peroxide utilizes a surrogate vapor that does not decompose over time. The electrochemical sensor responds to the presence of the surrogate vapor in a known manner. In this regard, the response of the electrochemical sensor to a specific concentration of the surrogate vapor can be correlated to a response of the electrochemical sensor to the presence of a known concentration of vaporized hydrogen peroxide. A correlation method is required in order to use a surrogate vapor for calibrating an electrochemical sensor at a particular concentration of vaporized hydrogen peroxide. A drawback to using a surrogate vapor for calibration of an electrochemical sensor is that a single correlation method may be applicable only over a limited range of concentrations of vaporized hydrogen peroxide. As a result, multiple correlation methods may be required in order to calibrate a sensor across a range of vaporized hydrogen peroxide concentrations.
The present invention overcomes these and other problems by providing a method and apparatus for calibrating a sensor for sensing vaporized hydrogen peroxide by determining the concentration of hydrogen peroxide in an aqueous solution of hydrogen peroxide.